mirror of https://github.com/irmen/ksim65.git
86 lines
2.5 KiB
Kotlin
86 lines
2.5 KiB
Kotlin
package razorvine.ksim65.components
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import razorvine.ksim65.Cpu6502
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/**
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* A programmable timer. Causes an IRQ or NMI at specified 24-bits clock cycle intervals.
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*
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* reg. value
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* ---- --------------
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* 00 control register bit 0=enable bit 1=nmi (instead of irq)
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* 01 24 bits interval value, bits 0-7 (lo)
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* 02 24 bits interval value, bits 8-15 (mid)
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* 03 24 bits interval value, bits 16-23 (hi)
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*/
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class Timer(startAddress: Address, endAddress: Address, val cpu: Cpu6502) : MemMappedComponent(startAddress, endAddress) {
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private var counter: Int = 0
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private var interval: Int = 0
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private var nmi = false
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private var enabled = false
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set(value) {
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if (value && !field) {
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// timer is set to enabled (was disabled) - reset the counter
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counter = 0
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}
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field = value
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}
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init {
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require(endAddress - startAddress + 1 == 4) { "timer needs exactly 4 memory bytes" }
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}
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override fun clock() {
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if (enabled && interval > 0) {
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counter++
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if (counter == interval) {
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if (nmi)
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cpu.nmi(this)
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else
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cpu.irq(this)
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counter = 0
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}
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}
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}
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override fun reset() {
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counter = 0
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interval = 0
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enabled = false
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nmi = false
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}
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override operator fun get(address: Address): UByte {
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return when (address - startAddress) {
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0x00 -> {
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var data = 0
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if (enabled) data = data or 0b00000001
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if (nmi) data = data or 0b00000010
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data.toShort()
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}
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0x01 -> (counter and 0xff).toShort()
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0x02 -> ((counter ushr 8) and 0xff).toShort()
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0x03 -> ((counter ushr 16) and 0xff).toShort()
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else -> 0xff
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}
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}
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override operator fun set(address: Address, data: UByte) {
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when (address - startAddress) {
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0x00 -> {
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val i = data.toInt()
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enabled = (i and 0b00000001) != 0
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nmi = (i and 0b00000010) != 0
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}
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0x01 -> {
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interval = (interval and 0x7fffff00) or data.toInt()
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}
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0x02 -> {
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interval = (interval and 0x7fff00ff) or (data.toInt() shl 8)
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}
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0x03 -> {
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interval = (interval and 0x7f00ffff) or (data.toInt() shl 16)
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}
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}
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}
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}
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